1. Field of the invention
The present invention relates to an improvement to electron tubes, more particularly power electron tubes operating at frequencies of the order of a few hundred megahertz.
The present invention relates more particularly to a means for internally cooling the structure supporting the cathode of the tube.
2. Description of the prior art
As shown schematically in FIG. 1 which relates to a power tetrode, the electron tubes to which the present invention applies are vacuum tubes formed essentially by cylindrical coaxial electrodes comprising an anode 1, a screen grid 2 called grid G.sub.2, a control grid 3 called grid G.sub.1 and a cathode 4.
These different electrodes are connected to the outside of the tube through circular metal connections 5, 6, 7, 8 separated from each other by insulators 9, 10, 11, 12 formed preferably from ceramic material and providing in addition sealing of the tube. These metal connections 5, 6, 7, 8 are in general formed by pieces stamped in the shape of cups and are brazed to the insulators.
The metal connections are connected to different voltage sources not shown and serve respectively for the passage of the heating current for the cathode and for the circulation of the high frequency currents.
However, heating of the cathode and circulation of the high frequency currents are heat generators and this heat is removed by conduction towards the metal connections.
Usually, the connections are cooled by injecting compressed air at the head of the tube. In most cases, this cooling is sufficient for maintaining the connections and the brazing of these connections to the insulators at a sufficiently low temperature which does not damage them.
However, the ultra high frequency operation of this type of tube gives rise to a sinusoidal distribution of the electric surface currents. Consequently, some zones of these surfaces which correspond to a current "antinode" where the intensity is maximum, are subjected to intense local heating.
In some cases of operation, these current antinodes are situated at the level of the connections. Consequently, French patent application No. 81 21804 has proposed a cooling system outside the tube formed by a spiral pipe through which flows a cooling fluid and in engagement, preferably by welding, with the connection of the electrode to be cooled.
With this cooling system, a considerable amount of heat is eliminated, in particular in the vicinity of a current "antinode".
However, the heating zones due to current "antinodes" situated inside the tube are not cooled . Now, in some cases, the heating is such that it brings the metal parts up to a high temperature, the resistance to the passage of current increasing the temperature. Thus, the increase in temperature may be such that the vapor tensions of the metals from which the electrodes are formed are reached. In this case, there is an emission of gas which results in at least a local deterioration of the vacuum and renders the tube unserviceable.
Furthermore, in power electron tubes of known type, the cylindrical shaped cathode comprises a sleeve formed by a network of crossed wires made preferably from thorium coated tungsten. This cylindrical sleeve is connected to the output connection through at least a skirt made from a refractory material such as tantalum, molybdenum or similar. Now, there also exist current "antinodes" on said skirt, which "antinodes" furnish heat which is added to that coming from the cathode. This heat is very difficult to remove outside the tube, for the connections are generally made from an ion-nickel-cobalt alloy which is a very poor heat conductor and of a small thickness so as to be readily brazed to the contiguous ceramic insulators.
Depending on the position of the current "antinodes" excessive heating of one end of the cathode may occur, which may cause localized deformations of said cathode which are harmful to the useful life of the tube.